1. Field of the Invention
This invention relates to the joining of superalloy articles by brazing, and, more particularly, to brazing alloys formulated as alloy powder mixtures.
2. Description of Prior Art
A variety of high temperature superalloys of the nickel-base and cobalt-base types are used in a variety of components in the high temperature operating section of gas turbine engines. Because gas turbine engines can be made to operate more efficiently by raising internal operating temperatures, a number of improved superalloys for use at progressively higher temperatures have been developed. Not only have new alloys been developed, but improved manufacturing techniques, such as directional solidification, which produce articles where the deleterious effect of grain boundaries on high temperature strength and creep resistance has been significantly reduced, have also been developed. However, where the design of parts or components for such engines requires that the superalloys be brazed, the temperature capability of the brazing alloy effectively limits the temperature capability of the brazed assembly. Thus, there has been a need for brazing alloys which retain their useful strength to higher temperatures, to better utilize the temperature capability of advanced superalloys, particularly those superalloys produced as directionally solidified or single grain articles.
Historically, brazing alloys formulated for joining superalloy articles have been categorized as precious metal alloys or nickel-base and cobalt-base alloys. Precious metal alloys are generally not usable at temperatures above about 1500.degree. F., and also suffer from the disadvantage of high material cost. Nickel-base brazing alloys typically contain nickel, a melting point depressant such as silicon or boron, and, in some alloys, some of the same alloying elements employed in superalloy articles, such as cobalt, chromium, aluminum, titanium, etc. The composition of one such brazing alloy, sometimes known as B-28, is the same as that of an alloy frequently used for conventionally cast turbine airfoils, called Rene'80, to which about 2 percent boron has been added. The brazing temperature for B-28 alloy is about 2215.degree. F.; its maximum effective use temperature is about 2000.degree. F.
Redden (U.S. Pat. No. 3,482,967) and Yount et al. (U.S. Pat. No. 3,542,543) have described nickel-base brazing alloys which include silicon as the only melting point depressant. These alloys were specifically formulated to avoid deleterious interaction between the molten brazing alloy and the substrate, particularly a substrate of the dispersion strengthened nickel-chromium type. The normal brazing temperature for the alloy described by Redden is approximately 2375.degree. F.; its maximum use temperature is about 2200.degree. F.
Several inventors, including Paulonis et al. (U.S. Pat. No. 3,678,570), Duvall et al. (U.S. Pat. Nos. 4,008,844 and 4,073,639), Baladjanian et al. (U.S. Pat. No. 4,285,459), Smith, Jr. et al. (U.S. Pat. No. 4,381,944) and Ferrigno et al. (U.S. Pat. No. 4,830,934), have described brazing materials which are mixtures of at least two different powder constituents having different melting temperatures. These mixtures achieve joining because the low-melting constituent liquefies at the brazing temperature, and wets both the substrate materials and particles of the high-melting constituent. Melting point depressants in the low-melting constituent, especially boron, can diffuse into both the substrate and the high-melting particles, thereby achieving isothermal solidification. Such materials are useful both for joining, and for filling voids like cracks or notches in superalloy articles. The size of the voids that can be filled with such materials is much greater than those which can be filled with conventional brazing alloys. However, such materials have been heretofore formulated to be brazed at temperatures lower than about 2215.degree. F. The maximum effective use temperature for articles brazed with such materials is about 2100.degree. F., or below. While these brazing materials are useful for joining conventional superalloys such as Rene'80, using them to braze advanced superalloys effectively precludes utilizing the full temperature capabilities of the advanced alloys.
The brazing temperatures used in the prior art have been at or below about 2200.degree. F. for two principal reasons. First, conventional superalloys such as Rene'80 are subject to incipient melting at higher temperatures. Second, it has been heretofore considered necessary that single grain superalloy articles be brazed at or below a temperature at which a sufficient amount of the gamma-prime phase remains undissolved in the gamma matrix so that recrystallization, which causes formation of stray grains in single grain articles, can be prevented. Precipitation of particles of the gamma-prime phase is the principal strengthening mechanism in superalloys of the type discussed herein.
Thus, there is a need for a braze material for brazing superalloy substrates which permits the brazed substrates to be used at design temperatures of the superalloy.